Tuesday, 4 August 2015

Over recent years, measles has once again become a public health crisis in the western world as a result of growing anti-vaccination movements in the UK and the US. This is an enormous problem because the success of vaccination depends on herd immunity: unless the vast majority of the population is protected, vaccines cannot halt the rapid spread of infectious diseases, leaving vulnerable people at risk, such as those with immune disorders, the elderly and newborn babies.

Efforts to encourage vaccine uptake have been hampered by a curious psychological phenomenon: the Backfire Effect. Research by Brendan Nyhan and Jason Reifler published this year demonstrated that efforts to persuade people to vaccinate their children can backfire, and actually reduce intentions to vaccinate. Now a new study, published yesterday in PNAS, has investigated further, adapting Nyhan and Reifler’s methodology.

A core problem in combating anti-vaccine views is that it is very hard to prove absence of risk, much in the same way that it is impossible to prove there isn't a cosmic teapot orbiting the sun. The authors of the new study, led by Zachary Horne, hypothesised that a better approach would therefore be to focus on convincing parents that without vaccinating, the risk of contracting vaccine-preventable diseases is high and the consequences severe.

First a quick re-cap on an earlier 2014 study by Nyhan and colleagues that provided the inspiration for the new research. Nyhan’s group compared the effects of four different types of evidence: 1) Correcting misinformation about the MMR vaccine; 2) Information about the risks of measles, mumps and rubella; 3) Parents’s reports on their children being infected with these diseases; and 4) Images of children infected with the diseases.

Nyhan's team found that none of these forms of evidence alone increased parents’ intentions to vaccinate their children. The effects of the information about measles, mumps and rubella were surprisingly neutral, while the images of infected children and the mother’s narrative about her hospitalised child both had the unintended effect of increasing beliefs in vaccine side-effects. The images also managed to inadvertently increase false beliefs that vaccines cause autism. The material that refuted the MMR-autism link successfully reduced false beliefs, but also reduced the intent to vaccinate in parents with the most anti-vaccine beliefs (i.e. there was a backfire effect). The overall picture was complex to say the least.

For the new study, Horne and his team combined the different information presented in the 2014 research to create a “disease risks condition” that included a mother's narrative of their child experiencing infection, alongside information about the effects of the diseases, as well as the images of the infected children. The comparison condition provided evidence contradicting the myth that MMR causes autism, and a control group read information that was irrelevant. Participants were hundreds of parents and non-parents surveyed online.

Contrary to the 2014 study, Horne’s team found that informing participants of the risks of failing to vaccinate did have a meaningful effect on parents’ beliefs about vaccines. This compared favourably to the information that debunked false beliefs about autism and the MMR vaccine, the effect of which did not differ significantly from the control condition. In this study neither intervention led to a backfire effect.

The results are encouraging, and indicate that a backfire effect is not unavoidable when communicating information about vaccine safety. The study is, however, not without its limitations. The study didn't directly evaluate intention to vaccinate – this is an important factor, as Nyhan’s 2014 study found that debunking false beliefs successfully reduced misperceptions about vaccines, but at the same time had the effect of reducing intentions to vaccinate in the very same people! In their 2014 paper they even remarked "if we had not measured intent, we might have missed a potentially dangerous backfire effect". The same researchers replicated this finding earlier this year in the context of flu. It seems a design flaw of the new Horne et al. study that this effect wasn't evaluated.

In response to the paper, Nyhan and Reifler remarked: "We’re excited to see the Horne et al. PNAS article on the effect of messages promoting vaccines and hope to see more experimental studies like this in the future. In addition, we’re encouraged that they find messages about the dangers of communicable diseases are effective in improving vaccine attitudes. Future research should consider the extent to which design differences between the two studies help explain the somewhat different conclusions the two studies reach, which will help us reach our shared goal of better understanding vaccine attitudes and behavior."

While the new research sounds like good news all round, celebration may be premature. The findings don't even get close to the bottom of the mystery that is the backfire effect; they don't explain the complexities teased out in earlier research, and they don't show that behaviour to vaccinate or not vaccinate would have changed. At a time when anti-vaccine beliefs are making a comeback, understanding and avoiding the backfire effect is more important than ever.

The next step for researchers will likely involve working out how we can make a powerful enough argument to convince the hard core of people with fixed anti-vaccine beliefs to vaccinate their children. The final step, and perhaps most difficult of all, will be ensuring that the change in beliefs stands the test of time.
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